Optical memory technology utilizing high-density and large-capacity optical storage media having a pit-formed pattern has been applied to a digital audio disc, a video disc, a document file disc and further a data file, etc. During the 1980s, compact discs (CD) that record and reproduce information by irradiation with light having a wavelength of about 780 nm became commercially practical, and during the 1990s, digital versatile discs (DVD) became commercially practical, which can record and reproduce higher density and larger amounts of information than the CDs by irradiation with light having a wavelength of about 650 nm. Both of these are used commonly today.
In the above optical memory technology, information is recorded and reproduced with respect to an optical storage medium by a minutely collected light beam. The accuracy and reliability of these recording and reproduction operations depend critically on the accuracy and reliability of an optical head device. Essential functions of the optical head device are roughly classified into: a function of converging light output from a light source to a minute spot diameter on the order of the diffraction limit; and a function of detecting a signal required for focus control so as to maintain a light spot on the optical storage medium, a signal required for tracking control so as to position the light spot at a midpoint of a specific track and a pit signal.
Meanwhile, one element constituting the optical head device is a photodetector. The photodetector receives light reflected from the optical storage medium, converts it into an electric signal (photoelectric conversion), detects an information signal recorded on the optical storage medium (hereinafter referred to as “RF signal”), a focus error signal (hereinafter referred to as “FE signal”), a tracking error signal (hereinafter referred to as “TE signal”) and the like, which are signals required for recording and reproducing, and outputs these signals. The photodetector also is used for receiving a part of the light emitted from the light source so as to control an output from the light source.
The above-described photodetector generally conducts the photoelectric conversion using a semiconductor with a photoelectric conversion area and, if required, a circuit attached thereto built therein. In order to carry out reliable recording and reproducing, needless to say, the photodetector also is required to have high reliability.
FIG. 7 shows one example of a conventional photodetector. A semiconductor chip 51 is secured onto a lead frame 54, and an electrode on the semiconductor chip 51 and a lead of the lead frame 54 are connected electrically via a bonding wire 55. The lead frame 54 is a terminal for inputting/outputting electric signals and electric power, and the photodetector is connected electrically with a flexible printed wiring board (not illustrated) and the like via the lead frame 54. The semiconductor chip 51, the bonding wire 55 and a part of the lead frame 54 are encapsulated within a resin body 52 having a light-transmitting property. The resin body 52 protects the bonding wire 55, a junction between the semiconductor chip 51 and the bonding wire 55, a surface of the semiconductor chip 51 with a circuit and the like built therein, and so on in order for these portions not to break due to a shock during the handling.
Light 56 reflected from the optical storage medium and containing a signal component having information recorded on the optical storage medium and the like passes through the resin body 52 and reaches a photoelectric conversion area 51a on the semiconductor chip 51 to undergo the photoelectric conversion. The photoelectrically converted signal travels through the bonding wire 55 to be output from the lead frame 54 as an electric signal. To this end, a material having a necessary transparency with respect to the light 56 and a favorable moldability, e.g., an epoxy resin is used as the resin body 52 (See JP S63(1988)-830 A, pp 1 to 2, FIG. 6, for example).
In recent years, an optical storage medium that allows recording and reproducing of still higher-density and larger-capacity information than that in DVDs has been developed, with the intention of shortening a wavelength of a light source used for the recording and reproducing of information with respect to such an optical storage medium from a red light source (wavelength of about 660 nm) to a blue light source (wavelength of about 400 nm). However, when a wavelength of light used for the recording and reproducing with respect to an optical storage medium is changed to, for example, about 400 nm, a light transmission area 61 of the light-transmissive resin 52 in the photodetector shown in FIG. 7 will be deformed gradually over a few hours to a few hundred hours due to the light incident on the photodetector. This adversely affects an optical path of the light passing through the light transmission area 61, thus making it impossible for the light reflected from the optical storage medium to reach the photoelectric conversion area 51a on the semiconductor chip 51 while maintaining a correct profile. As a result, the photodetector cannot detect desired electric signals such as a FE signal and a TE signal sufficiently. Therefore, an optical information processing device using an optical head device including such a photodetector has a problem of the failure in adequate operations by a focus control unit and a track control unit.
In addition, there is another problem of a decrease in the amplitude of a RF signal, which impairs the reliability of reproducing.
Also in a photodetector that has a function of detecting a signal used for judging the magnitude of a light amount to control an output from a light source and outputting the signal, if the resin body 52 is deformed significantly, a part of the light does not reach the photoelectric conversion area 51a due to reflection, diffraction and the like. This leads to a problem of the failure in the precise detection of the above-stated signals.